There is a great need for devices or apparatuses which make it possible to identify or detect objects over a certain distance without making contact. In addition, a need exists to be able to change the data stored in, or operating characteristics of, these devices or apparatuses (e.g., "program" the devices or apparatuses).
It is, for example, desirable to request, over a certain distance, identifications which are uniquely assigned to an object. These identifications could be stored in the device or apparatus so that, for example, the object may be identified. A determination may also be made as to whether or not a particular object exists within a given reading range.
As another example, physical parameters such as temperature or pressure can be interrogated directly even when direct contact to the object is not possible. A device or apparatus of the type desired can, for example, be attached to an animal which can then always be identified at an interrogation point without direct contact.
There is also a need for a device which, when carried by a person, permits access checking whereby only persons whose responder unit returns certain identification data to the interrogation unit are allowed access to a specific area. In this case the safeguarding of the data transfer is a very essential factor in the production of such devices.
A further example of a case in which such a device is needed is the computer controlled industrial production in which, without the intervention of operating personnel, components are taken from a store, transported to a production location and there assembled to give a finished product. In this case a device is required which can be attached to the individual components so that the components can be specifically detected in the spares store and taken therefrom.
Several transponder arrangements have been developed. One such transponder arrangement is described in U.S. Pat. No. 5,053,774 issued on Oct. 1, 1991, incorporated herein by reference. This patent describes a transponder unit which has a low energy requirement and does not need its own power source.
Each of these systems, as well as others, requires an antenna for each of the interrogation unit and transponder to transmit and receive signals. Therefore, there is a need for antennas with a high quality factor (Q), light weight, and manufacturable at a low cost.
One air coil 10 which has previously been used as an antenna is illustrated in FIG. 1. The air coil 10 is formed from a cylindrical wire 12 which is wound around a core 14, typically an air gap. The prior art air coil 10, however, has a number of problems. For example, these air coils have a lower Q than is desirable. In addition, smaller and lighter antennas are desired.